They've always been terrible at animation. 10 or 20 years ago, their animations have always been the absolute worst by any contemporary standard (the art used to be too - see Battlespire for some terrible art - but they improved it). Maybe part of that was because of the engine, but I think they just never had the culture for it.

They clearly did try to improve their animations in Fallout 4 in 2013-2014, which is the timeframe the most development happened, so it's not like they're oblivious to their biggest shortcoming as a studio. So what they did in F76 and Starfield is just a regression.